Fig. 4.

Capping pathway and enzymes in relation to the proteome of nidoviruses. The conventional mRNA capping pathway is shown on the left, with the enzymes catalyzing the respective four reactions listed in bold. Further to the right, presence of these enzymes in viruses of five nidovirus families, each designated by its prefix, is listed (see Fig. 3 for phylogeny and pp1ab domain organization). RTPase, 5′-triphosphotase; GTase, guanylyl transferase; N-MT, guanine-N7-methyltransferase; O-MT, 2′-O-methyltransferase. In ^m7GpppN_2’-Om notation, ^m7G stands for 7-methylguanosine, p stands for phosphate, N_2’-Om stands for the 5′-terminal nucleoside of the RNA molecule, methylated at the ribose-2′-O position. For details, see text.